The present invention is directed to a new and improved method and apparatus for removing volatile components from liquid materials, especially liquid materials which have foaming characteristics, are heat-sensitive, or both.
In the chemical, food, pharmaceutical and other industries, it is often necessary to strip or evaporate certain volatile components from a liquid material to obtain a final commercial product. For example, solvents and/or monomers used in an efficient process for manufacturing a chemical product must be removed from the product at the end of the chemical process. Of course, any commercially acceptable evaporation process must remove the volatile components from the liquid product as efficiently as possible with little or no deleterious effects upon the final concentrated liquid. In the case of heat-sensitive, foam-forming liquids, the evaporation process becomes of critical importance in that the liquid must be efficiently concentrated to commercially acceptable levels with little or no foaming (which creates practical problems in handling the material) and with heat applications to the liquid which will not damage the heat-sensitive product.
Typically, volatile components are evaporated from a liquid material in accordance with the well known batch vessel process. Pursuant to the batch vessel process, the liquid material is placed in a large generally cylindrical vessel which includes a steam pipe extending into the interior of the vessel. The steam pipe is provided with steam outlet means submerged in the liquid material whereby steam may be continuously injected into the liquid material. The steam will flow through the material and rise to the top of the vessel. As the steam flows through the liquid material it will heat and evaporate the volatile components contained in the liquid material. The rising steam and evaporated components are collected and removed from the top of the vessel. In order to strip the volatile components from the liquid material whereby less than 0.2% of the volatile components remain in the final liquid product, it is necessary to leave the liquid product in the vessel, exposed to continuous steam flow for a period of between 20 to 24 hours. Consequently, the prior art batch vessel process is inefficient in that it requires a substantial amount of time to effectively strip volatile components from a liquid product. Moreover, the liquid material generally sustains some damage by virtue of being exposed to a constant steam flow for a substantial length of time. This is especially true in the processing of heat-sensitive liquids.
As a consequence of the special problems associated with heat-sensitive liquid products, those skilled in the art have sought to develop improved evaporation techniques ideally suited to such products. One prior art solution, disclosed in U.S. Pat. No. 3,073,380, comprises the steps of passing the material to be concentrated through a heated tortuous flow path of relatively narrow elongated cross-section at a high velocity. The flow path is arranged such that the value of the heat exchange areas of the flow path as measured in square feet far exceeds the value of the volume of liquid flowing through the flow path as measured in cubic feet. Pursuant to the process, sufficient heat is applied to the liquid path to vaporize the volatile components of the liquid as the liquid flows through the path to form a homogeneous mixture of concentrated liquid and vapor. The flow of the homogeneous mixture is continued through the tortuous path while additional heat is added to the mixture.
As indicated in U.S. Pat. No. 3,073,380, the disclosed tortuous flow path induces a turbulent flow of the material to achieve an excellent heat transfer thereby raising the temperature of the liquid rapidly to the point where vaporization of the volatile components occurs. The abrupt changes of flow direction caused by the tortuous flow path and the high velocity of the fluid flow cause a high degree of evaporation. Moreover, in the case of foam-forming liquids, such evaporation occurs without foaming. The material leaves the tortuous flow path as a concentrated liquid carried in homogeneous mixture by the vapors formed in the process. Moreover, inasmuch as vapor is generated continuously within the tortuous flow path, sensible heat is continuously converted to latent heat of vaporization to thereby reduce the temperature of the product, and avoid damage to the heat-sensitive liquid material.
While the disclosure of the above-discussed U.S. Pat. No. 3,073,380 provided the art with a significant advance for concentrating heat-sensitive and foam-forming liquid products, the method disclosed was not entirely satisfactory for stripping volatile components from a liquid material whereby only very small quantities of the volatile component remain in the liquid after processing. To that end, U.S. Pat. No. 3,469,617 disclosed an improvement of the method covered by the earlier patent. More particularly, the U.S. Pat. No. 3,469,617 proposed initiating a high velocity continuous phase of stripping vapor flow, e.g., steam, through the tortuous flow path and adding to the vapor flow immediately upstream from the inlet port to the tortuous flow path the material to be stripped in an appropriate weight proportion to the vapor. The tortuous flow path induces a turbulent flow causing new surfaces of the liquid material to be continuously exposed to contact with the stripping gas of the vapor phase to thereby continuously strip volatile substances from the liquid material.
Pursuant to the process of U.S. Pat. No. 3,469,617, the vapor flow is used to form a continuum for the liquid material to be stripped and as the source of heat for the stripping process. This is in contrast to the earlier discussed U.S. patent wherein the vapor continuum is gradually formed by the vaporization of the volatile components of the liquid material and heat is applied by external means. The use of a continuous vapor phase from an outside source which is combined with the liquid material prior to passage through the tortuous flow path greatly increased the amount of vaporization occurring in the volatile components of the liquid material, thus achieving a stripping action to substantially eliminate such volatile components from the liquid product. However, a disadvantage of the proposed method disclosed in U.S. Pat. No. 3,469,617 is that the use of a vapor continuum requires additional apparatus structure to provide and transport the stripping gas to the tortuous flow path as well as special vapor inlet pipesto properly mix the incoming stripping gas and liquid to be stripped. The introduction of hot gas directly into the liquid material at the inlet pipe results in local overheating which may adversely effect product quality. Moreover, the overall capacity of the tortuous flow path must be sufficient to accommodate the additional volume requirements of using a stripping gas from an outside source which is mixed with the liquid product prior to ingress of the resulting mixture into the tortuous flow path.
From the foregoing, it should be understood that while those skilled in the art have provided means for concentrating liquid materials and stripping volatile components from such products, certain disadvantages are inherent in the prior proposals. More specifically, the batch vessel process is inefficient timewise and generally unsuitable for delicate liquid products. The earlier proposal of U.S. Pat. No. 3,073,380 while providing an excellent means for concentrating delicate liquid materials, was not suitable for a stripping operation. The vapor continuum was formed solely by gradual vaporization of the volatile components of the liquid material throughout the flow path and therefore sufficient volume for the vapor phase as required in a stripping operation could not be readily achieved. As discussed above, the process of the later U.S. Pat. No. 3,469,617 improved the efficacy of the method disclosed by the earlier patent, however, not without the need of additional apparatus structure, increased volume requirements for the tortuous flow path and localized heating of the liquid flow.
It is a primary objective of the present invention to provide a new and improved process and apparatus for evaporating volatile components from a liquid material which retains the advantages of the earlier proposals while eliminating the disadvantages. Generally, the invention comprises the steps of passing the liquid product to be stripped or concentrated through a heating means in which sufficient heat is added to the liquid to achieve vaporization of the volatile components, yet maintaining the pressure, volume and velocity conditions of the liquid within the heating means at predetermined levels such that no vaporization occurs while the liquid is passing through the heating means. Thereafter the heated liquid product is passed through a restrictive orifice-forming flow path into a tortuous flow path stripping section. The restrictive orifice-forming flow path is arranged to maintain the proper pressure conditions within the heating means to prevent any vaporization from occurring therein. Moreover, the volume, pressure and velocity conditions within the tortuous flow path stripping section are maintained at certain predetermined levels such that relative to the conditions existing within the restrictive orifice-forming flow path, the heated liquid product will "flash" as it flows from the restrictive orifice-forming flow path into the tortuous flow path stripping section. The flashing action brought about by the relatively larger volume due to pressure differences will cause rapid vaporization of the heated volatile components to rapidly form a vapor continuum flowing at an extremely high velocity. The vaporization being caused by a pressure drop will occur at relatively low temperatures to greatly minimize the possibility of a local overheating of the liquid material. The vapor continuum will carry the liquid in atomized droplet form.
Pursuant to a significant feature of the invention, the tortuous flow path stripping section is of a relatively narrow elongated cross section with a high ratio of surface area to volume of fluid flow. Thus, inasmuch as the overall volume of the stripping section is much greater than the volume of the restrictive orifice-forming flow path to effect flashing, the fluid flow will be turbulent facilitating a rapid vaporization of the volatile components to form a vapor phase continuum. Moreover, the vapor phase continuum formed pursuant to the process steps of the invention will act to essentially atomize the liquid into droplets suspended in the vapor phase. Each droplet will be turbulently moved through the flow path to provide a high degree of exposure of each droplet to the vapor whereby the remaining volatile components are evaporated from the droplet. The fluid flow leaving the tortuous flow path stripping section will be in the form of a high velocity vapor spray including finely atomized droplets of liquid substantially stripped of volatile components. This exiting fluid flow may be passed into a separator wherein the finely atomized liquid droplets will condense and collect at the bottom of the separator apparatus while the vapor phase rises to and is removed from the top portions of the separator.
It has been found that the process according to the present invention may be carried out in a most effective and economical manner by utilizing a novel arrangement of a plate-type evaporator including multiple fluid passes with each pass having a relatively high pass surface area to enclosed volume ratio. Pursuant to a feature of the apparatus aspects of the inventive concept, the several fluid passes of the plate-type evaporator are arranged into three functional sections. The first section will operate as the heating unit and includes several heated passes through which the fluid is passed. The fluid is heated to a temperature below the boiling point of the volatile components to be removed at the existing pressure conditions within the first section of the plate-type evaporator. The second section will operate as the tortuous flow path stripping section and comprises most of the remaining passes of the plate-type evaporator. Generally, these passes will be unheated except that under certain circumstances some controlled heating of the stripping section may be used to maintain adiabatic conditions within the stripping section without the introduction of direct, live steam. The number of fluid passes within the stripping section will usually exceed the number of passes in the first heated section to permit the necessary volume expansion of the heated liquid as it flows from the heated section to the stripping section to obtain flashing of the liquid and vaporization of the volatile components. In the plate-type evaporator embodiment of the invention, a predetermined number of passes, typically one, are arranged between the heated section and stripping section to act as the restrictive orifice-forming flow path to maintain the proper pressure conditions within the heated section and to provide the necessary structure to cause a flashing of the liquid as it flows from the heated section to the stripping section.
While it is most convenient and economical to utilize the first several passes of a plate-type evaporator as the heated section, any type of suitable pre-heater of tubular or similar construction may be used as the heating means of the process. Such a pre-heater unit will be arranged upstream from the plate-type stripping section of the evaporator whereby the liquid product will flow into the pre-heater for proper heating and immediately thereafter flow into the inlet port of the plate-type stripping section of the evaporator for stripping, as discussed above. Of course, in all instances, a constriction of fluid flow must be provided between the pre-heater and the plate-type evaporator to maintain the proper pressure conditions within the pre-heater and to effect the flashing of the liquid product into the plate-type evaporator. It should be noted that the tortuous flow path stripping section may comprise, as an alternative to the plate-type evaporator, any structure arranged to induce a turbulent fluid flow as the liquid product is flashed from the heating means. For example, the fluid may be passed into an axially elongated tube including an internal corrugated metal structure to substantially occupy the internal spaces of the tube to define a plurality of narrow, elongated, turn-inducing flow paths for the fluid flow.
The present invention provides the art with a significant improvement by teaching a novel means and apparatus for effectively removing volatile components from a liquid product with a controlled heat application sufficient to bring about vaporization of the volatile components without damaging the liquid. The process contemplates a high velocity for the liquid whereby the liquid is exposed to the heating section for a minimal amount of time and immediately flows into the stripping section whereby the added heat is quickly converted to latent heat of vaporization. Moreover, the flashing aspects of the process bring about rapid vaporization of the volatile components to form an atomized spray with the liquid being suspended in the vapor continuum formed by the vaporized components in atomized droplet form. The atomized spray continues to flow through the tortuous flow path whereby the droplets undergo further stripping. The sequence of steps taught by the present invention achieves excellent results within a straightforward, space-saving apparatus structure and without the need of a stripping gas, such as steam or the addition of heat to the stripping section. The more specific features of the invention will be determined by the particular characteristics of the liquid product, the degree of stripping or evaporation of volatile components desired as well as the sheer tolerance, thermal stability and foaming characteristics of the liquid material in process, as will appear.
For a better understanding of the above and other features and advantages of the present invention, reference should be made to the following detailed description of preferred embodiments of the invention, results of test runs of the process, and the accompanying drawings.